Printed circuit (or wiring) boards are well known in the electronics field. In general, such boards consist of a dielectric resin impregnated substrate (e.g. of woven or non-woven glass fibers) that is adhered to a sheet or foil of conductive metal, generally copper, on at least one surface. Typical resins that can be used to impregnate the substrate include phenolic resins, epoxy resins, polyimides, polyesters and the like. The copper sheet or foil is joined to the semi-cured resin impregnated substrate using well known techniques, e.g. by application of heat and pressure.
Thereafter, electrical circuit patterns are formed on the copper layers by conventional techniques. For example, a layer of photoresist may be coated over the copper layer, exposed imagewise and developed to yield a relief resist image on the copper layer. The exposed copper is etched away and the resist image is removed, leaving the copper circuit pattern exposed. Elemental copper, like other pure metals, generally exhibits poor adhesion characteristics for bonding to dielectric resinous substrates typically used in circuit board manufacture, and intermediate conversion coatings are frequently helpful to enhance the adhesion of the metal to the substrate. Hence, the copper foil may be treated prior to being laminated to the resin substrate to form a layer of copper oxide, tin or other adhesion promoter on at least one surface. Various methods have been employed for this purpose. Examples of such methods are described in U.S. Pat. Nos. 2,955,974; 3,177,103; and 3,198,672.
Printed circuit boards prepared as described above may be assembled to form multilayer printed circuit board constructions by stacking a predetermined number of boards one atop another. In such a construction, the cured or semi-cured polymeric non-conductive materials (such as epoxy resin impregnated fiberglass cloth) is in contact with the copper surface of the adjacent circuit board. The stacked circuit board assembly may be laminated together by application of heat and pressure to form a multilayer printed circuit board.
Typical laminating conditions involve pressing the stacked boards between metallic plates at a pressure between about 200 psi to about 600 psi at a temperature of between about 150.degree. C. to 205.degree. C. for up to about 4 hours. The electrical circuit patterns on the outer layers may then be interconnected to the circuit patterns on the inner layers by drilling an array of holes through the multilayer assembly circuit boards. The through-holes in the assembled boards are then cleaned by treatment with dilute solutions of strong acids and the like. Thereafter, the through-holes may be plated with copper to render the sides of the holes conductive thereby completing the circuit between the outer layer and the inner layer circuit patterns.
Black copper oxide (CuO) coatings have been employed for a number of years to promote adhesion between the etched copper circuit patterns and intermediate polymeric dielectric layers of multilayer boards. In general, such coatings are prepared by oxidizing the exposed surface of the copper with an oxidizing agent. However, problems with this approach have prompted a search for improved coatings. In particular, total or local delamination of the copper circuit pattern layer from the polymeric layer may occur due to poor adhesion and/or the tendency of the copper oxide layer to dissolve upon exposure to acidic solutions employed in subsequent treatment of the multilayer board. The combination of poor adhesion and susceptibility of the copper oxide to dissolution upon exposure to such solutions is known as the "pink-ring" or "halo" effect.
The pink-ring effect occurs primarily in the vicinity of the circuit through-holes that pass through two or more layers of a multilayer printed circuit board assembly. These so-called "through-holes" are subsequently made conductive by plating with copper metal or filling with solder to become part of the electrical circuit constructed on the printed circuit boards. Prior to the plating or soldering operation, but subsequent to lamination and drilling, the through-holes are cleaned to remove debris from drilling (e.g. resin smear) with a cleansing solution comprising a dilute aqueous solution of a strong acid (e.g. sulfuric acid, hydrofluoric acid, chromic acid and the like). This cleansing operation tends to cause dissolution of the copper oxide coating exposed by the through-holes. This can later result in delamination or loss of adhesion between adjacent metal and plastic layers in the vicinity of the through-holes. Upon close inspection, a pink-ring appears to be formed in the area around the hole. Appearance of this pink-ring phenomenon is believed to be evidence of delamination of the metal/plastic layer in the printed circuit board in which such effect is observed.
A number of approaches to solving the problem of pink-ring have been tried. For example, thicker coatings of copper oxide have been deposited on the exposed copper surface to retard the dissolution process by sheer volume of the copper oxide remaining after dissolution (see for example U.S. Pat. Nos. 2,955,974; 3,177,103; and 4,409,037). However, the thicker coatings result in poor initial adhesion between the coated metal and resinous substrate even before the cleansing solution is applied, and therefore do not solve the problem resulting from the pink-ring effect. Another approach has been to use a coating of tin in place of the copper oxide layer as an adhesion promoter. However, the level of adhesion achieved with this technique has not been sufficient to overcome the problem (U.S. Pat. Nos. 4,732,649 and 4,775,444).
Other approaches to solving the problem of pink-ring include methods of treating copper oxide-coated copper surfaces to promote adhesion between said copper surfaces and resin impregnated substrates. U.S. Pat. Nos. 4,661,417; 3,585,010; and 3,293,109 disclose methods to electrolytically reduce copper oxide coated surfaces to metallic copper or electrolytically deposit copper metal onto metallic copper surfaces prior to joining such surfaces to plastic substrates. These electrolytic methods, however, are not useful in printed circuit boards having disconnected elements, i.e. circuits that are not electrically connected to one another since the disconnected elements cannot be electrically connected to an electrical power source as required in performing an electrolytic reduction.
A variety of reducing agents known in the art, e.g. formaldehyde, sodium hypophosphite, hydrazine and the like, can reduce cupric compounds in aqueous solution to metallic copper. Such reductants are used for metallizing plastic surfaces and either do not appreciably reduce surface copper oxide to metallic copper or result in the formation of a powdery copper surface (see U.S. Pat. No. 4,642,161 for a general discussion of reducing agent limitations). For example, U.S. Pat. No. 4,642,161 discloses a method of reducing copper oxide coatings to metallic copper using reducing solutions containing simple amine boranes. However, treatment of copper oxide surfaces with amine boranes has been determined to produce powdery copper coatings that are unacceptable for use in electrical circuit boards.
U.S. Pat. No. 4,997,516 discloses a method for reducing copper oxide coatings to metallic copper using an alkaline reducing solution containing sodium dithionite and polymeric addition agents such as gelatin and glue. However, treatment of copper oxide surfaces with sodium dithionite produces a mass of metallic copper whiskers by a primary reduction process and amorphous cuprous oxide which quickly converts to copper powder in a concurrent secondary reduction process. The copper powder cannot be separated from the copper whiskers and a significant amount of secondary reduction must be interrupted by rapid treatment of the copper surface with an acid-salt wash to dissolve the cuprous oxide and thus prevent formation of copper powder. This method is impractical because the timing of the acid-salt wash must be carefully balanced between allowing sufficient reduction of black oxide to metallic whiskers and preventing secondary formation of substantial amounts of copper powder.
Despite these efforts, a simple, cost-effective process which promotes strong adhesion between adjacent copper foils and polymeric substrates which imparts superior mechanical, thermal and electrical properties and avoids the delamination and loss of adhesion attendant to the so-called pink-ring effect has not been available until the advent of the present invention.